Engineering plants for pollution control, biofuels

Yesterday's release of articles by PNAS contained three that focus on …

Even people who aren't environmentalists would probably agree with two contentions: we need to find a replacement for fossil fuels, and we've made a bit of a mess in terms of using toxic chemicals that are difficult to remove and dispose of safely. This week, the journal PNAS will be releasing a number of papers that suggest the solutions to these two unrelated problems may be linked, although it's not necessarily a solution that all environmentalists will embrace: genetically modified plants. Two articles explore the use of plants to eliminate toxic chemicals, while a third explores using plants to produce hydrogen. We'll look at each individually.

Growing on leftover ammunition: One of the great examples of evolution has been the appearance of bacteria that can digest the toxic and explosive compound hexa-hydro-1,3,5-trinitro-1,3,5-triazine, commonly termed RDX. RDX doesn't occur naturally in significant amounts, but has contaminated soil and groundwater at military installations and ammunitions plants. Given high levels of this chemical, bacteria have evolved a series of enzymes that break it down in order to obtain organic nitrogen from it. Unfortunately, those bacteria that can digest RDX aren't sufficiently common to actually eliminate it or protect groundwater.

Reasoning that plants have more elaborate needs than bacteria and can reach a higher biomass, the authors took two of the genes in the bacterial RDX pathway and inserted them into Arabidopsis, a standard lab plant. The resulting plants could clear a 150 µMolar solution of RDX in less than a day.

Trees for removing organic carcinogens: A second paper takes a look at the problem of removing a series of toxic organic compounds from the environment. Apparently, mammals have a gene that produces a protein (cytochrome P450 2E1) that breaks down trichloroethylene, vinyl chloride, carbon tetrachloride, chloroform, and benzene, while the plant version of the gene is less efficient. So, the research team simply placed the mammalian version of P450 into a DNA construct that ensured high levels of expression in plants, and inserted that DNA into the genome of the poplar tree. The resulting trees could clear several of these compounds, either from the air or from hydroponic solutions, in a matter of a week.

The poplar isn't exactly ideal for all circumstances. It's deciduous, meaning that most exchange with the air shuts down for the winter. It's good in terms of going through a lot of water (meaning it can purify more), but that's bad in terms of fresh water being a limited commodity. Still, the method should be applicable to just about any plant, so it's possible that this could be customized—appropriate species could be engineered according to the specific environmental needs.

Getting Chlamydomonas to make hydrogen: In covering the sequencing of the Chlamy genome, I mentioned that it was a candidate for biofuels manufacture. The new publications shows how that might work. Chlamy can produce hydrogen when it switches to alternative energy pathways when there is light, but no oxygen. There are two problems with this: Photosystem II in the chloroplasts normally produces oxygen from water when provided with light, and these alternate pathways ultimately leave Chlamy short of carbon, killing it.

The authors found a gene regulatory element (called a promoter) that only comes on under two conditions: oxygen or copper starvation. They hooked this promoter up to a gene that's needed to make Photosystem II, and then placed the cells in a copper-rich solution. This shut the promoter off; the cells gradually ran out of Photosystem II, and then burned through their remaining oxygen over the course of six hours. Once the oxygen was gone, they began producing hydrogen. At the same time, the oxygen starvation kicked the promoter back on, making more Photosystem II, thus providing enough oxygen to keep Chlamy from burning through all its carbon sources.

Although they didn't present data demonstrating it, the authors suggest that this system should cycle between oxygen production and hydrogen production, keeping the cells alive while still producing some useful fuel. They also make a few suggestions as to how further genetic manipulation of Chlamy might increase the efficiency of hydrogen production. If they could just get Chlamy to grow well in salt water, biofuels would start looking like a serious replacement for fossil fuels.